Sensor for measuring the electrical conductivity of a fluid medium

ABSTRACT

The invention relates to an inductively operating sensor ( 1 ) for measuring the electrical conductivity of a fluid medium ( 2 ), having an excitation coil ( 3 ), to which an input signal is fed, and a receiver coil ( 4 ) coupled with the former via the fluid medium ( 2 ), which provides an output signal (I Ind ), which is a measurement for the conductivity of the fluid medium ( 2 ). To make possible the timely detection of damage to the windings of the excitation coil ( 3 ) or the receiver coil ( 4 ), or respectively a service cable for the sensor ( 1 ), which leads to leak currents or short circuits, it is proposed by the invention that the sensor ( 1 ) has means ( 5 ) for measuring a variable signal at the input of the excitation coil ( 3 ). The excitation coil ( 3 ) of the sensor is preferably fed by an input voltage (U Err ), and the means ( 5 ) for measuring the variable signal preferably measure the input current (I Err ) at the input of the excitation coil ( 3 ).

FIELD OF THE INVENTION

[0001] The present invention relates to an inductively operating sensorfor measuring the electrical conductivity of a fluid medium, having anexcitation coil, to which an input signal is fed, and a receiver coilcoupled with the former via the fluid medium, which provides an outputsignal, which is a measurement for the conductivity of the fluid medium.

[0002] Such sensors have an excitation coil which is for exampledesigned as a toroid coil and is fed by an a.c. voltage. A ring-shapedmagnetic alternating field is generated in the interior of theexcitation coil. A receiver coil, which can also be designed as a toroidcoil, is arranged at the same level in which the excitation coil alsolies. Because of the magnetic alternating field in the excitation coil,mobile ions in the fluid medium generate a ring-shaped current in thefluid medium to be measured, which in turn triggers an output signal inthe receiver coil, whose strength is a function of the mobility andconcentration of the ions, and therefore of the electrical conductivityof the fluid medium. Customarily the output signal appears as aninduction current.

[0003] Sensors of this type are preferably employed in the food or drugindustry for monitoring the production processes in productioninstallations for producing food or drugs. The sensors must alwaysprovide an accurate and dependable output signal, so that excessivechanges in the conductivity of the medium to be measured can be rapidlydetected and a correspondingly rapid reaction takes place in order to beable to prevent deterioration of the food or drugs to be produced. Theappropriate reactions to a change in conductivity can be triggeredeither indirectly by the production crews, or directly by the productioninstallations.

[0004] In the course of its employment, the sensor can be exposed tostrong mechanical and thermal stresses. Because of this, damage to thewindings of the excitation coil or the receiver coil can occur in somecases. Leak currents, or even short circuits, can occur between thedamaged windings. The output signal can be distorted because of the leakcurrents between the windings, and a short circuit between the windingsrenders the entire sensor unusable.

[0005] Moreover, because of the mechanical or thermal stresses on thesensor, a short circuit or a break in the service cable for the sensorcan occur. It is clear that because of this, the output signal can alsobe distorted, or respectively the entire sensor can become unusable.

[0006] Initially, the distorted signal is not detected as such by theproduction personnel, or respectively the production installation.Initially, the production personnel, or respectively the productioninstallation assumes that the output signal has detected a changedconductivity of the medium to be measured and reacts accordingly bymatching the production processes to the new conductivity values of themedium. Only after some time, or respectively in case of a considerablydistorted output signal, will it be possible to detect, for example bymeans of a probability check, that the output signal is distorted, orrespectively that the sensor is defective.

[0007] Normally, production is continued during this time. Changes inthe production process can occur because of the distorted output signal,which over time can lead to the production of a defective product. Thiscan lead to the necessity of destroying the entire running productionbatch for safety reasons in order to dependably preclude anyendangerment of the health of the purchasers because of defective foodor drugs. This entails considerable costs. In accordance with the priorart it is not possible at all, or respectively possible much too late,to detect damage to the windings of the excitation coil or the receivercoil, or respectively the service cable of the sensor.

OBJECT AND SUMMARY OF THE INVENTION

[0008] The present invention is therefore based on the object ofdesigning and further developing a sensor of the type mentioned at theoutset in such a way that it allows an early detection of damage to thewindings of the excitation coil or the receiver coil, or respectively ofthe service cable, which could lead to leak currents or short circuits.

[0009] For the attainment of this object the invention proposes, basedon a sensor of the type mentioned at the outset, that the sensor hasmeans for measuring a variable signal at the input of the excitationcoil.

[0010] It was noted in accordance with the invention that, in case ofthe appearance of leak currents or short circuits, damage to thewindings of the excitation coil or receiver coil results in a drasticincrease in a variable signal at the input of the excitation coil. Ifthe input signal is in the form of a voltage, the input current at theinput of the excitation coil will increase as a result of the damage tothe sensor. In this case the means provided will measure the inputcurrent. If the input signal is in the form of a current, the inputvoltage at the input of the excitation coil will rise because of thedamage. In this case the means provided will measure the input voltage.

[0011] This signal at the input of the excitation coil acts in the sameway in case of damage to the service cable of the sensor, which mightlead to leak currents or short circuits. The variable signal at theinput of the excitation coil thus provides rapid and dependableinformation regarding the ability of the sensor to function. Damage tothe windings of the excitation coil or the receiver coil, orrespectively to the service cable of the sensor, which result in leakcurrents or short circuits, can be detected early and dependably bymonitoring this signal at the input of the excitation coil.

[0012] The production crew can react without delay to such a detectedsensor defect. For example, production can initially be stopped in orderto prevent the production of defective products. The defective sensorcan be exchanged for a new one, and production can then be startedagain. In addition, it would also be possible to perform a measurementcheck of the conductivity of the medium to be measured in order to checkwhether the sensor is actually defective. The shut-off and subsequentrestart of production can also be performed directly by the controldevice of the production installation, without the production crewhaving an input on this.

[0013] It is proposed in accordance with an advantageous furtherdevelopment of the invention that the sensor has a voltage source, whichfeeds an input voltage to the excitation coil, and that the means formeasuring the variable signal pick up the input current at the input ofthe excitation coil.

[0014] Preferably the means for measuring the input current have amultiplier and measure the voltage dropping across the multiplier. Sincethe voltage changes proportionally with the input current, it ispossible to determine the input current at a sensor designed in this wayin a simple manner.

[0015] In accordance with another advantageous further development ofthe present invention it is proposed that the sensor has a measuredvalue transducer for receiving the output signal, which is connectedwith the means for measuring the variable signal at the input of theexcitation coil, that the means generate a status signal, which is afunction of the measured value of the variable signal at the input ofthe excitation coil, and that the means feed the status signal to themeasured value transducer. In this way, during the measurement operationthe measured value transducer is always aware of the ability of thesensor to function. The status signal lies within a defined thresholdrange as long as the sensor functions. However, if the monitoredvariable signal at the input of the excitation coil steeply increases asa result of damage, the means for measuring the variable signal generatean appropriate status signal which lies outside of the threshold range.The measured value transducer can appropriately react without a timedelay to such a status signal, from which it determines the lack of theability of the sensor to function. As a reaction, the measured valuetransducer can stop the entire production, for example, so that aproduction of defective products does not even occur.

[0016] In accordance with a preferred embodiment of the presentinvention it is proposed that the measured value transducer corrects theoutput signal as a function of the strength of the status signal. Ifdamage to the windings of the excitation coil or receiver coil, orrespectively of the service cable of the sensor, only results in aslight distortion of the output signal, this will also lead to a smallchange of the variable signal at the output of the excitation coil. Themeasured value transducer can react to such a change in the variablesignal for example with a corresponding correction of the output signal.By means of this it is possible to continue the assurance of thefunction free of defects of the sensor.

[0017] In accordance with a further preferred embodiment, the measuredvalue transducer causes a signal to be issued if the status signal liesoutside of a defined threshold range. This report can be merely used toinform the production crew, which can then react accordingly. However,this report can also have the character of an alarm signal, whichautomatically triggers defined reactions, or respectively stops theproduction installation.

[0018] A preferred exemplary embodiment of the present invention will beexplained in greater detail in what follows, making reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 shows a sensor in accordance with the invention in apreferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] An inductively operating sensor in accordance with the inventionis identified as a whole by 1 in FIG. 1. The sensor 1 is used formeasuring the electric conductivity of a fluid medium 2. The sensor 1has an excitation coil 3 designed as a toroid coil, which is fed by analternating voltage U_(Err). A ring-shaped alternating magnetic field isgenerated in the interior of the excitation coil 3. A receiver coil 4 isalso arranged on the same level on which the excitation coil 3 islocated and is also designed as a toroid coil. A ring-shaped currentI_(Med) is generated in the fluid medium 2 to be measured by ions movingin the fluid medium 2 because of the alternating magnetic field in theexcitation coil 3, which in turn triggers an induction current I_(Ind)in the receiver coil 4. The strength of the induction current I_(Ind) isa function of the mobility and concentration of the ions and thereforeof the electric conductivity of the fluid medium 2.

[0021] The sensor 1 has means for measuring the input current I_(Err),which are identified as a whole by the reference numeral 5. The means 5for measuring the input current I_(Err) have a multiplier R and measurethe voltage U dropping across the multiplier R. Damage to the windingsof the excitation coil 3 or the receiver coil 4, or respectively to theservice cable (not represented) of the sensor 1, which result in leakcurrents or short circuits, can be detected early and dependably bymonitoring the input current I_(Err) of the excitation coil 3.

[0022] It is conceivable for the sensor 1 to have a measured valuetransducer (not represented) for receiving the induction currentI_(Ind), with which the means 5 for measuring the input current I_(Err)are connected. The means 5 for measuring the input current I_(Err)generate a status signal, which is a function of the measured value ofthe input current I_(Err) and which is supplied by the means to themeasured value transducer. Now the measured value transducer cancorrect, for example, the induction current I_(Ind) as a function of thestrength of the status signal, so that an error-free function of thesensor 1 is assured in spite of damage to the sensor 1. But the measuredvalue transducer can also cause the issue of an alarm signal, if thestatus signal lies outside of a defined threshold range.

What is claimed is:
 1. An inductively operating sensor (1) for measuringthe electrical conductivity of a fluid medium (2), having an excitationcoil (3), to which an input signal is fed, and a receiver coil (4)coupled with the former via the fluid medium (2), which provides anoutput signal (I_(Ind)), which is a measurement for the conductivity ofthe fluid medium (2), characterized in that the sensor (1) has means (5)for measuring a variable signal at the input of the excitation coil (3).2. The sensor (1) in accordance with claim 1, characterized in that thesensor (1) has a voltage source, which feeds an input voltage (U_(Err))to the excitation coil (3), and that the means (5) for measuring thevariable signal pick up the input current (I_(Err)) at the input of theexcitation coil (3).
 3. The sensor (1) in accordance with claim 2,characterized in that the means (5) for measuring the input current(I_(Err)) have a multiplier (R) and measure the voltage (U) droppingacross the multiplier (R).
 4. The sensor (1) in accordance with one ofclaims 1 to 3, characterized in that the sensor (1) has a measured valuetransducer for receiving the output signal (I_(Ind)), which is connectedwith the means (5) for measuring the variable signal at the input of theexcitation coil (3), that the means (5) generate a status signal, whichis a function of the measured value of the variable signal at the inputof the excitation coil (3), and that the means (5) feed the statussignal to the measured value transducer.
 5. The sensor (1) in accordancewith claim 4, characterized in that the measured value transducercorrects the output signal (I_(Ind)) as a function of the strength ofthe status signal.
 6. The sensor (1) in accordance with claim 4 or 5,characterized in that the measured value transducer causes a signal tobe issued, if the status signal lies outside of a defined thresholdrange.